1. Field of the Invention
The present invention relates to a dual-band circularly polarized antenna and, more particularly, to a dual-band circularly polarized antenna that can transmit and receive circularly polarized signals at two different frequency bands simultaneously.
2. Description of Related Art
The antenna modules used in certain fields of application, such as the antenna module of an RFID system, of a base station of a mobile communication system or of a satellite communication system, are required to transmit and receive circularly polarized signals in two different frequency bands simultaneously. However, the circularly polarized antenna that are currently being used only can transmit and receive circularly-polarized signals in one single frequency band at a time, rather than in two different frequency bands. Although some kinds of existing dual-band antennas can transmit signals in two frequency bands simultaneously, the signals transmitted can only have the characteristics of linear polarization. That is, the existing dual band antenna is still unable to transmit and receive circularly polarized signals in two frequency bands at the same time. As a result, the existing type of dual-band antenna can only serve as a ‘dual-band linear polarized antenna’ rather than a ‘dual-band circularly polarized antenna’.
FIG. 1A is a perspective view of a prior art dual-band linear polarized antenna, which is able to transmit and receive linear polarized signals simultaneously within the UHF band (between 860 MHz and 930 MHz) and the microwave band (between 2.45 GHz and 2.55 GHz). The antenna comprises a first linear polarized radiating element 11, four second linear polarized radiating elements 121, 122, 123, 124 and a ground plate 13. For brevity, the first microwave substrate on which the first linear polarized radiating element 11 is formed and the second microwave substrate on where the second linear polarized radiating elements 121, 122, 123, 124 are formed are omitted in the figure.
Both the first microwave substrate and the second microwave substrate are made of an FR-4 microwave substrate, and both the first microwave substrate and the second microwave substrates are 1.6 mm thick. To assemble the prior art dual-band linear polarized antenna, the first microwave substrate shall be placed on the surface of the second microwave substrate. Then, the second microwave substrate and the first microwave substrate thereon are placed on the upper surface 131 of the ground plate 13 made of metallic material, as shown in FIG. 1B.
As shown in FIG. 1A, the straight-line shaped signal coupling unit 14 is electrically connected with the first linear polarized radiating element 11 and an adapter 15, and the prior art dual-band linear polarized antenna is electrically connected with a signal processing unit (not shown) by a coaxial cable (not shown) being attached to the adapter 15. As a result, the prior art dual-band linear polarized antenna can transform an electrical signal from the signal processing unit (not shown) into two linear polarized signals at two respective frequency bands. The two linear polarized signals are then transmitted into the ambient environment (transmitting state). On the other hand, the prior art dual-band linear polarized antenna can receive linear polarized signals of any kind of frequency bands and transform these signals into corresponding electrical signals. The electrical signals are then provided to the signal processing unit for subsequent signal processing steps (receiving state). That is, the prior art dual-band linear polarized antenna can neither transmit nor receive any circularly polarized signals.
The industry, therefore, is still in the search for a dual-band circularly polarized antenna, which is able to transmit and receive circularly polarized signals simultaneously at two different frequency bands for improving the efficiency and widening the range of applications that an antenna module could offer.